1. Field of the Invention
The present invention relates generally to an air conditioning system for an automotive vehicle. More particularly, the present invention relates to a method and system for controlling an automotive heating, ventilating and air conditioning system to prevent fogging.
2. Disclosure Information
A fundamental goal of automotive heating, ventilating and air conditioning (HVAC) systems is to provide comfort to vehicle occupants as well as to avoid window fogging conditions. During normal air conditioning operation, moisture retention on the evaporator and within the evaporator case of the HVAC system has been the cause for fogging within the vehicle cabin. Additionally, moisture brought into the vehicle as a result of natural human functions such as breathing, perspiration, wet umbrellas, snow on clothing, etc., all contribute to the overall moisture content within the passenger cabin of the vehicle. In general, interior window fog elimination is simply a process of dehumidifying the cabin by operating the air conditioning system and further by warming this "dry air" and distributing it to the effected glass areas.
The process of keeping the vehicle glass fog free for general operating conditions is understood and currently controlled by the operator of a vehicle in an open loop fashion. For example, it is commonly known to turn the air conditioning on or activate the defrost mode (which also activates the air conditioning in many vehicles) when fogging is visible. Improvements to existing systems have been suggested, such as in U.S. Pat. No. 5,516,041, assigned to the assignee of the present invention and herein incorporated by reference. The system of the '041 patent closes the known open loop with a humidity sensor which provides input to an algorithm which calculates fog probability and automatically takes fog preventive correction actions. These processes work well as intended, however, they are limited to "normal" air conditioning operating conditions.
Presently, the typical air conditioning system as described above controls window defogging only for temperatures down to approximately 42.degree. F. Normally, at temperatures below 42.degree. F., the air conditioning operation is deactivated to prevent a wet evaporator core from freezing. A frozen evaporator core will block air flow through the ducting system of the air conditioning system.
However, the greatest probability of fogging occurs in the spring and fall of the year between temperatures of 25.degree.-45.degree. F. During this time of year, a low pressure switch in the A/C system keeps the A/C system off, offering no dehumidification and when the vehicle heater is in the medium to full operation, the fogging probability is high. As the passenger cabin warms up, it has a greater capacity to hold moisture and the interior relative humidity increases. If the dew point is reached, moisture will form on the cooler surfaces within the passenger cabin, normally the inside of the glass which is exposed to a cooler exterior ambient temperature. Interior relative humidity can increase significantly during these conditions and are further increased when the operator brings moisture into the cabin in the form of a wet raincoat or clothing covered with snow. This combination of warming the cabin, melting snow and a cold ambient temperature is a perfect condition for fogging to occur. In current air conditioning systems, the air conditioning does not operate at the ambient temperatures at which these conditions typically occur.
Therefore, it would be advantageous to modify an air conditioning system to provide for air conditioning operation during the times of high fogging probability and yet prevent freezing of the evaporator within the system. Furthermore, it would be advantage to provide such a system at relatively low cost with no equipment changes to the present air conditioning system. It is an object of the present invention to provide such a system and a method for preventing fogging during these conditions.